mem: Add const getters for write packet data

This patch takes a first step in tightening up how we use the data
pointer in write packets. A const getter is added for the pointer
itself (getConstPtr), and a number of member functions are also made
const accordingly. In a range of places throughout the memory system
the new member is used.

The patch also removes the unused isReadWrite function.

src/cpu/simple/atomic.cc

@@ -469,7 +469,7 @@ AtomicSimpleCPU::writeMem(uint8_t *data, unsigned size,
 
                 if (req->isSwap()) {
                     assert(res);
-                    memcpy(res, pkt.getPtr<uint8_t>(), fullSize);
+                    memcpy(res, pkt.getConstPtr<uint8_t>(), fullSize);
                 }
             }